The present invention relates to improvements in or relating to a multi-level, band-restricted waveform generator for generating a band-restricted waveform of a multi-level signal sequence, which is employed to produce various multi-level modulated signal waveforms of a multi-value amplitude, a multi-level phase and a multi-level frequency in a modem for data transmission over a wire or radio line with band limitation.
There have heretofore been used an analog or digital filtering method which directly applies the multi-level square wave sequence signal to an analog or a digital low-pass filter for band restriction, and a waveform table ROM method according to which, with respect to all combinations of k symbols (k being impulse response symbol lengths), the waveform corresponding to the k-th symbol after the band restriction is precalculated on off line basis and prestored in a ROM (Read Only Memory) as a waveform table and is then read out therefrom for D/A conversion.
With the analog or digital filtering method, however, the low-pass filter for band restriction use is usually high-order, and hence is large in the scale of its circuit structure, and when the low-pass filter is formed by an analog element, it calls for compensation for an environmental change and time aging and fine adjustment and is not suitable for fabrication as an IC. Thus, the low-pass filter has difficulties in miniaturization, economization and stabilization. If the low-pass filter is implemented by a digital signal processing device as a digital filter through utilization of A/D and D/A conversion techniques, its stabilization can be achieved but its circuit scale and power consumption will be large.
On the other hand, the digital processing method utilizing the waveform table ROM is generally excellent in terms of miniaturization, economization and stabilization, but letting the number of samples per symbol be represented by S, the capacity of the ROM (i.e. the number of memorized words for D/A conversion) is S.multidot.M.sup.k words. Hence, as k and M increase, the capacity becomes enormous, and consequently, this method is impractical when k is comparably large.